1. Field of the Invention
This invention relates to an adhesive composition for a metal foil to be used for preparation of a laminated plate for a printed wiring board to be used for an electric and electronic equipment, an adhesive-coated metal foil, a metal-clad laminate, a wiring board, a multi-layer board and a multi-layer wiring board using the same.
2. Background Art
Electronic equipment for public use becomes a small size and has high functions so that a printed wiring board to be used therein is required to be high density and fine wiring pattern. Accompanying with this tendency, a copper-clad laminate to be used in a printed wiring board is required to be high density when it is actually fit. Thus, required levels of solder heat resistance or copper foil-peel resistance is becoming severer. Also, for electric equipment to which a high voltage is applied such as a television set, tracking resistance is now required to ensure safety. Tracking means a phenomenon, which forms a carbonized conducting path or a tracking path between the portions having a difference in voltage on the surface of an insulating material.
In the prior art, a copper clad laminated board has been prepared by adhering an adhesive-coated metal foil in which an adhesive layer is formed on a copper foil and a prepreg in order to improve adhesiveness between the copper foil of the copper clad laminated board and the prepreg. As the adhesive, an adhesive in which a phenol resin is formulated into a polyvinyl acetal resin has been used since it is excellent in the points of adhesiveness, solder heat resistance and copper foil peel resistance.
However, in an adhesive in which a phenol resin is formulated into a polyvinyl acetal resin, whereas solder heat resistance, etc. are excellent, but the phenol resin is easily carbonized so that the adhesive becomes easily conductive whereby it is inferior in tracking resistance.
Thus, to improve tracking resistance of an adhesive, a method of using an epoxy resin or a melamine resin has been proposed, for example, in Japanese Provisional Patent Publication No. 116682/1987.
However, only when an epoxy resin or a melamine resin is formulated into an adhesive, the tracking resistance cannot sufficiently be improved. And yet, as compared with the case where a phenol resin is formulated, reactivity or compatibility of a resin is markedly different so that heat resistance can hardly be maintained.